This invention relates to a relay apparatus, a network, and a frame relay method and, in particular, to technologies for working around a loop on the network and for ensuring redundancy in a case where a plurality of relay apparatuses are connected in a loop fashion to compose the network.
In a case where a plurality of relay apparatuses are connected to a network, there are ring redundancy methods as technologies for working around a loop on the network and for ensuring redundancy. One of the ring redundancy methods is disclosed in an informal paper under the title of “Extreme Network' Ethernet (registered trademark) Automatic Protection Switching (EAPS) Version 1”, RFC (Request For Comments) 3619 (October 2003).
However, in a case of carrying out a path switching in the ring redundancy method, there is a possibility that reversal of arrival order of a frame occurs and it results in having a large effect on frame transfer of a higher layer. In addition, when the reversal of the arrival order of the frame occurs, an improper learning occurs in a MAC (Media Access Control) address table and there is a possibility that communication cannot be carried out after occurrence of the reversal of the arrival order.
In a case where a plurality of relay apparatuses are connected in a ring-shaped fashion by using a ring redundancy method, a control protocol is required to release a loop of a frame relay path. However, a current control protocol may occur reversal of arrival order of a frame in a transient state on a path switching and may have a deleterious effect on frame transfer of an upper layer. In addition, when the reversal of the arrival order occurs, a mistaken learning of the MAC address table occurs. It is therefore a possibility that it is impossible to carry out communication on or after occurrence of the reversal of the arrival order.
When a failure occurs in a link of a part in ring structure of the Ethernet (registered trademark), a phenomenon in which arrival order of the frames reverses occurs. By the reversal of the arrival order, it gives rise to problems as follows:
(1) it has adverse effect on the upper layer because the order of data reverses; and
(2) when a frame arrives through an old route after a frame arrives through a new route and a route learning is carried out, the old route is mistakenly learned in the address learning table and there is a possibility that it compromises communications after this.
Various arts related to this invention are already proposed. By way of illustration, Japanese Unexamined Patent Application Publication of Tokkai No. 2001-127,782 or JP-A 2001-127782 (which will be also called a first patent document), which corresponds to U.S. Pat. No. 6,952,396 discloses a control system which enables transport of payload data across a dual counter rotating ring (DCRR) network having two or more nodes. Each node provides access to the DCRR network by a respective local area network (LAN) subtending the node. The control system comprises a topology learning entity operative within each node for monitoring individual, links of the DCRR, discovering the active topology of the DCCR, communicating topology changes to other nodes in the DCCR, and informing frame forwarding and learning processes of the topology to enable failover to redundant resource upon detection of a network component failure. A frame forwarding process selects a shortest path route between a source and a destination node in the DCCR, and forwards frames to the destination node via the selected shortest path route. An address learning process inspects a source medium access control (MAC) address in each received frame to learn a shortest path route for each MAC address.
In addition, Japanese Unexamined Patent Application Publication of Tokkai No. 2001-308893 or JP-A 2001-308893 (which will later be called a second patent document) discloses a routing information dynamic reconfiguration system in a loop topology network that automatically performs routing of a substitute path, on the occurrence of a fault in a communication cable and a node. The loop topology network disclosed in JP-A 2001-308893 comprises first through fourth nodes which are connected in form of a loop. The first and the fourth nodes comprise first and fourth network interface cards (NICs) which detect a fault of a third communication cable between the first and the fourth nodes, and inform first and fourth control software items of the first and the fourth nodes about the result of detection. The first and the fourth control software items inform the second and the third nodes about fault information. The second and the third nodes eliminate all routing information via the third communication cable, transmit an alive notice packet denoting the normality of the own nodes to the first and the fourth nodes. When the first and the fourth nodes are marked as being a communication disabled state in the information of the packet, the first and the fourth nodes add the information of their own nodes to the information of the packet, transmit the resulting information to an opposite node to reconfigure the routing information to a target node.
Japanese Unexamined Patent Application Publication of Tokkai No. 2004-147172 or JP-A 2004-147172 (which will later be called a third patent document), which corresponds to U.S. Patent Application Publication No. 2004/0081082, discloses a ring switchover method which enables a fast ring switchover in a packet processing at the time of ring configuration. In the third patent document, a system switchover function can be implemented in an Ethernet switch or the like by the ring switchover method in a network constituted of layer 2 switches connected to a ring shape. Each layer 2 switch has a path control function and a failure detection function. The ring switchover method includes the steps of providing in each layer 2 switch and address learning table in which a Media Access Control (MAC) address and a corresponding port are stored; on detecting of a link failure between mutually neighboring layer 2 switches, transmitting a failure notification frame packet from each neighboring layer 2 switch; and in the layer 2 switch having received the failure notification frame, recording a Media Access Control (MAC) address of said layer 2 switch into the failure notification frame, and transferring the failure notification frame to a neighboring layer 2 switch.
Japanese Unexamined Patent Application Publication of Tokkai No. 2005-27,039 or JP-A 2005-27039 (which will later be called a fourth patent document) discloses a duplex ring network performing uninterruptible transmission which is equivalent to a line switching technology without affecting the transmission efficiency of a packet. The duplex ring network disclosed in the fourth patent document comprises first through seventh nodes which are connected in a ring-shaped fashion. When the first node transmits a unicast data packet to the second through the seventh nodes, the first node selects a ring with the shortest path. It will be assumed that the first node acts as a transmission source node while the sixth node acts as a destination node. When the failure occurs in the selected ring with the shortest path, the first node (the transmission source node) selects a ring that does not pass through a failure place. In this case, the transmission source node stores a unicast data packet for all nodes at a normal time ahead of the failure occurrence and transmits the stored unicast data packet the ring that does not pass through the failure place when the destination node (the sixth node) is not on the ring with the shortest path when the transmission source node selects the ring that does not pass through the failure place. Thus, the unicast data packet for the destination node can be transmitted through the second through the fifth nodes with no short break.
Japanese Unexamined Patent Application Publication of Tokkai No. 2005-252,672 or JP-A 2005-252672 (which will later be called a fifth patent document) discloses a switching hub. In this switching hub, no flooding occurs immediately, when switching a communication path by generating a barrier. JP-A 2005-252672 discloses a ring-shaped network comprising first through eighth switching hubs which are connected to a transmission path in a ring-shaped fashion. The first switching hub having a function for logically disconnecting a loop by logically blocking a clockwise side port thereof. It will be assumed that a failure occurs in the transmission path between the fourth and the fifth switching hub. In this event, the fourth switching hub re-registers fifth through eighth address learning information pieces into a clockwise side port of the opposite side. The fifth through the eighth address learning information pieces are transmitted from the clockwise side port of the fourth switching hub. When the third switching port receives the fifth through the eighth address learning information pieces, the fifth through the eighth address learning information pieces are re-registered to a clockwise side port of the opposite side.
However, any of the first through the fifth patent documents neither discloses nor teaches reversal of arrival order of a frame and mistaken learning of a MAC address learning table.